1. Field of the Invention
The present invention concerns a magnetic resonance apparatus with a gradient coil and an electrically conductive structure that interacts with the gradient field generated by the gradient coil.
2. Description of the Prior Art
Magnetic resonance is a known modality for acquiring images of the interior of an examined object. In a magnetic resonance apparatus rapidly switched gradient fields generated by a gradient coil system are superimposed to a static magnetic field generated by a basic field magnet. Furthermore, the magnetic resonance apparatus includes a radio-frequency system that emits radio-frequency signals into the examined object to generate magnetic resonance signals, and receives the resulting magnetic resonance signals, on the basis of which magnetic resonance images are generated.
In order to generate the gradient fields, electrical currents must be fed into the gradient coils of the gradient coil system. The amplitudes of the required currents can be as large as several hundred amperes. The current rise and fall rates are as large as several hundred kA/s. Since the gradient coil system is usually surrounded by electrically conductive structures, the switched gradient fields induce eddy currents in the gradient coils. Examples of such conductive structures are the vacuum chamber and/or the cold shields of a super-conducting magnet, a radio-frequency shield, made for example of a copper foil, and the antenna of the radio-frequency system. The fields accompanying the eddy currents are undesirable, because, if no counter-measures are taken, they weaken the gradient fields and distort them over time, which negatively affects the quality of the magnetic resonance images.
The distortion of a gradient field caused by eddy current fields can be compensated, to a certain degree, by a corresponding pre-distortion of a parameter that controls the gradient field. To achieve compensation, the controlling parameter must be filtered in such a manner that the eddy current fields that occur during operation of the gradient coil without pre-distortion are cancelled by the pre-distortion. This filtering can be done by a filter network with parameters that are determined by time constants and coefficients that can be ascertained, for example, by a method known from the patent German PS 198 59 501.
Moreover, the use of an actively shielded gradient coil system also can reduce the eddy currents induced by the gradient coil with current therein. Such shielding involves a presettable enveloping surface that extends, for example, through an inner cylinder surface of an 80-K cold shield of the super-conducting magnet. A secondary coil pertaining to the gradient coil usually has a lower number of turns than the relevant gradient coil, and is connected to the gradient coil in such a manner that the same current flows through the secondary coil as through the gradient coil, but in the opposite direction. The secondary coil weakens the gradient field in the volume to be imaged. A gradient coil with a corresponding secondary coil for the reduction of a gradient field on a predetermined enveloping surface is described, for example, in the patent British Specification 2 180 943.
In addition, German OS 34 11 222 describes a magnetic resonance apparatus that includes three gradient coils for the generation of gradient fields and at least one more coil arrangement that operates independently of the gradient coils for the generation of a magnetic field extending in the direction of the main magnetic field. The additional coil arrangement is designed so that the magnetic field changes spatially in a non-linear manner and so that the superposition of the magnetic field with gradient fields results in a defined, temporal and spatial change in the magnetic flux density. In one embodiment, the additional coil arrangement is designed so that the magnetic field has a spatial course corresponding to a spherical function of the second or third order. The additional coil arrangement can especially eliminate the undesirable eddy current effects caused by the gradient fields.
Finally, the patent German OS 101 56 770 discloses a magnetic resonance apparatus with a gradient coil system, in which an electrically conductive structure is arranged and designed so that, at least within the imaging volume of the magnetic resonance apparatus, a magnetic field generated by the electrically conductive structure due to the induction effects of the gradient field is similar (in a generic sense) to the gradient field. In a particular embodiment, at least a part of the structure is designed as a circular surface component of the magnet. This allows, among other advantages, a gradient coil system to be constructed without secondary coils, because the undesirable consequences of the switched gradient fields can be controlled by a pre-distortion due to the similarity of the magnetic field generated by the conductive structure, so that no weakening of the gradient fields occurs due to the secondary coils.